The useful lifetime of small and medium diameter (20 to 40 mm) gun barrels is limited by damage of the interior surfaces resulting from mechanical and thermochemical effects related to passing a projectile through the gun bore and subsequent exposure of the interior surface to hot propellant gases. Coatings to protect the interior surfaces of the barrel are therefore frequently employed. Traditionally, the gun barrels have contained chromium coatings that are applied on the interior surface via electroplating. These coatings provide adequate performance; unfortunately hexalavent chrome is created during the electrodeposition process. This material is toxic and difficult to dispose of. Executive Order EO13148 requires the usage reduction of hexavalent chrome (the primary element of electro-deposition) by 50% before the end of 2006. New deposition approaches for wear resistant coatings are therefore desired that retain the high throwing power and affordable cost structure of electroplated chrome but are inherently environmentally safe.
Several other deposition options for protective coatings currently exist. These include approaches such as thermal spraying, chemical vapor deposition (CVD) and the various physical vapor deposition (PVD) approaches. The internal surfaces, however, are hidden or limited from sight making the coating of internal surfaces with very uniform thickness distribution and a high quality microstructure very difficult or impossible. While high pressure CVD using metal-organic precursors may at first provide a promising approach, non-uniform deposition, vapor toxicity issues and low deposition rates plague this approach. Thus, the desired combination of non-line-of-sight coating capability, limited-line-of sight capability, high deposition uniformity, environmental inertness and compositional flexibility required has been difficult to achieve.
Perhaps the most promising approach is PVD. These approaches are growing in interest for many applications because they are environmentally friendly, allow adequate materials flexibility and enable the deposition of high quality, thin films. In most PVD based processing approaches, however, it is not possible to uniformly coat the interior of hollow tubular substrates without spatially distributed sources (such as cylindrical magnetron sputtering (CMS) where source targets are inserted into the interior of the part). This arises because the vapor atoms are created in a high vacuum that results in nearly collisionless vapor transport to the substrate. As a result, only regions in the line-of-sight of the vapor source are coated. Even for the cases of thin films deposited with cylindrical magnetron sputtering, deposition rates are relatively low and the vacuum requirements are stringent (<10−4 Pa) so that the cost effectiveness of these approaches in relation to electroplating is in question. In addition, the ability of these processes to deposit coatings into the grooves found in rifled gun barrels is also an issue. Nevertheless, this PVD approach still appears to be one option for coating large diameter gun barrels (>40 mm). Its application to smaller diameters, however, is not feasible because of issues related to the stability of the ionization and deposition processes involved.
Thus, the advent of a new deposition process that improves upon the economic and the line-of-sight limitations of current PVD approaches (such as CMS) while retaining their many advantages is of interest for applications such as gun barrel coatings, the coating of other tubular substrates and other components having an interior region or integral parts.
Other applications, for example and not limited thereto, that would benefit from such advancement include wear and corrosion resistant coatings for the interior surfaces of aircraft landing gear components, wear resistant coatings for actuators in suspension control systems used on automobiles, hydraulic and pneumatic actuators, linear electric motors and the internal surfaces of bearings.